[TenTec] Transmission Line Loss due to SWR

[Sinisa Hristov]

"Martin, AA6E" wrote:
 
> Don't these numbers apply to "long" lines (many wavelengths and/or an exact
> multiple of a quarter wavelength)?
 
The tables and the formula they are based on
(may be found in ARRL Antenna Book) are valid
for any length of line, and any degree of loss,
provided the line is uniform.


> I have been wondering about the following: If you have a short line, the
> loss should be different. That's because the loss is normally due to I^2R --
> it mostly arises where the current is maximum. If you have, say, a line less
> than 1/8 wavelength long feeding a high Z load, the loss should be rather
> small, because the entire line is high voltage / low current.

I was wondering just the same and had to perform
a short analysis to convince myself.

Let's take as an example a typical 50 Ohms line terminated
with 0.5 and 5000 Ohms loads. It looks like the SWR is the same
in both cases, i.e. 100. But it isn't, unless the line is lossless.

As there is some loss, Zc cannot be 50 Ohms, i.e. it must be complex.
even if all the loss is in the conductor and the dielectric is perfects.

When the actual complex Zc is used to calculate SWR,
the latter turns out to be vastly different for the two loads,
leading to vastly different and correct values of additional loss.


73,

Sinisa  YT1NT, VE3EA